The present invention is concerned with an improved process for preparing chromized steel. The invention is further concerned with a novel process for preparing chromized steel which utilizes an improved steel substrate. In still a further variant, the ivention is concerned with the improved chromized steel products prepared by the process of the invention.
Chromized steel has surface layers about 1 - 3 mils in thickness of an iron-chromium alloy on a low carbon steel substrate. The alloy is formed by diffusing chromium into the surface of the steel substrate. This may be conveniently accomplished by heat treating the steel substrate at an elevated chromizing temperature, in a protective chromizing atmosphere and in the presence of a source of chromium and a chromizing energizer until a desired thickness of the chromized coating is produced. The resulting chromized coating has corrosion characteristics comparable to the 400 series of stainless steel and especially types 409 and 430. The chromized steel product is highly corrosion resistant and may be used as a low cost substitute for stainless steel.
High quality chromized steel is characterized by a substantially continuous corrosion resistant coating of diffusion layer of iron-chromium alloy having a metallographically measured thickness of about 1 - 3 mils and preferably about 2 mils. The appearance of the chromized steel surface should be pleasing to the eye and and substantially uniform in color without mottling or staining of portions of the surface area. The chromized steel surface should be smooth and free of sintered particles which adhere tightly and produce an unsatisfactory rough surface. The chromized steel product also must have good mechanical characteristics. An ASTM ferrite grain size of 3 or finer, a transition temperature no higher than +20.degree. F., an elongation in two inches of at least 30%, a yield strength of at least 17,000 psi, a tensile strength of at least 35,000 psi, and excellent formability are very desirable. Formability is especially important as the chromized steel may be used to manufacture mufflers for automobiles and other corrosion resistant articles which require intricate forming operations. If the product does not have good formability with respect to both the iron-chromium alloy layer and the underlying substrate, then the corrosion resistant alloy layer is cracked during the forming step and the desirable corrosion resistant properties of the formed article are lost in the vicinity of the crack.
The chromized steel products produced heretofore have had the above described desirable characteristics to varying degrees. However, prior to the present invention, it was very difficult if not impossible to consistently produce an entirely satisfactory chromized steel product in a wide variety of thicknesses from the available steel substrates.
The present invention overcomes the above mentioned and other disadvantages of the prior art chromizing substrates by providing a plain low carbon columbium treated steel substrate having a controlled free or diffusable carbon content, columbium content and thickness which are carefully correlated as described more fully hereinafter. This substrate assures that a high quality chromized steel product possessing the above mentioned combination of desirable properties to a remarkable degree will be produced consistently with any desired substrate thickness.
It is an object of the present invention to provide an improved steel substrate which greatly improves the appearance and mechanical characteristics of chromized steel prepared therefrom.
It is a further object to provide a novel process for preparing chromized steel whereby the appearance and mechanical characteristics of the chromized steel product may be maintained at optimum levels and especially when employing a noncompacted coating of particulate chromium as the chromium source.
It is still a further object to provide the improved chromized steel products prepared in accordance with the chromizing process of the invention.
Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the illustrative examples.